It is known to measure the oxygen content of exhaust gases with the aid of a concentration cell, for example, made of ZrO.sub.2 ceramic, with respect to a reference oxygen content in a reference atmosphere. The reference atmosphere can, for example, be defined by the ambient air or also by a reference gas in a volume closed off or virtually closed off with respect to the ambient. A stable oxygen atmosphere is achieved in this volume, for example, in that oxygen is pumped out of the exhaust gas into the volume by impressing a current. Such a pump reference atmosphere affords the advantage with respect to an ambient air reference that an adulteration of the reference gas atmosphere is comparatively slight. Adulteration of the reference atmosphere can, for example, be caused by water spray or fuel vapor in direct proximity of the arrangement. In general, the reference volumes are selected to be very small and are not gas tight with respect to the ambient, for example, because of residual porosity of the ZrO.sub.2 ceramic or because gas channels are deliberately built in which are intended to avoid gas pressures in the reference volume which are too high. For this reason, a stable reference atmosphere can only be maintained with a permanently impressed current.
However, this is associated with the disadvantage that a decomposition of the electrolyte can occur because of the applied voltage which is needed. This decomposition of the electrolyte can especially occur when exhaust gas is produced from the combustion of a fuel-rich mixture and the exhaust gas has a correspondingly low content of oxygen.
Furthermore, a temperature-dependent change of the measuring signal occurs when the pump voltage is applied to the electrolyte via the measuring electrodes. The measuring signal Us is taken off from the measuring electrode at the exhaust-gas end and a reference electrode at the reference-gas end. In this case, the measuring signal Us is comprised additively from the Nernst voltage Un and a voltage excursion Ri*Ip. The Nernst voltage Un results because of the differing oxygen partial pressures at the electrolyte. Ri represents the internal resistance of the electrolyte and Ip identifies the pump current flowing as a consequence of the pump voltage. The product Ri*Ip essentially defines a temperature-dependent disturbance of the measurement signal because of the temperature dependence of the probe internal resistance Ri.